This invention relates to the field of two-way base-mobile-portable communications systems and more particularly to the prevention of repeater interference by the establishment of a "priority" repeater.
The use of two-way radio communications between base and mobile units for police, fire-fighting, forestry departments and the like is well known in the art. The problems caused by the need to return to the vehicle before giving or receiving information are also well known and have led to the widespread use of portable units which can be carried easily upon the person of the user. These latter units are generally low power transceivers which rely upon a mobile unit to repeat out with greater power on a different channel when communication with the base station is required. When a number of mobile units converge on a location, as is frequently the case in emergency situations, interference becomes a problem since each of the mobile units hears each portable transmission and repeats it out to the base. Each mobile unit would also repeat out any base station channel transmission received, including signals "skipped-in" from a distance. Among the partially successful attempts to solve the interference problem was the use of a subaudible tone code superimposed on the carrier signal wherein each mobile-portable pair would have its own code and would communicate only with its matching unit and with the base station. This solved part of the interference problem but care had to be exercised to keep matched pairs together, and if one unit was disabled its mate was useless and the vehicle was partially disabled as well. Another solution was to have the dispatcher coordinate all transmission in an area, allowing only one transmission at a time but this was a complicated and troublesome procedure.
A further solution may be found in an improved system as set forth and disclosed in a co-pending application, Ser. No. 590,006, and, assigned to the same assignee as the present invention, in which each vehicle contains a mobile transceiver, including a repeater, as well as a portable unit. In this system, each mobile unit arriving at a location would normally assume priority over all other mobile units present by transmitting a carrier burst which would cause each other unit to up-count (or further delay) its repeater. This would produce a prioritized stack of repeaters, ideally having one repeater in each incremental delay state beginning at zero. If, when a mobile arrived, there was already repeating in progress, that mobile would up-count its own repeater to the highest possible incremental delay state. Since there were possibilities of having more than one unit in a given state and/or some states vacant, there remained the possibility for brief, random delay before a message would be repeated out.